1. Field of the Industrial Utility
The present invention relates to an organic film having semiconductivity. electroconductivity or even superconductivity. More particularly, the present invention relates to an electroconductive organic thin film made of a vapor-deposited bisethylene dithiatetrathiafulvalene (BEDT-TTF) film doped with an electron acceptor, as well as a process for producing such an organic thin film.
Further, the present invention relates to a vapor-deposited BEDT-TTF film that can be used as an electroconductive organic thin film, as well as a process for producing such vapor-deposited film.
2. Description of Related Art
Organic thin films can be produced by various techniques such as a Langmuir-Blodgett (LB) method, vapor deposition, an ionized cluster beam (ICB) method and a molecular beam epitaxy (MBE) method. Using these methods, semiconductive or electroconductive organic thin films have been prepared from tetrathiofulvalene (TCNQ), tetrathiofluvalene-tetracyanoquinodimethane (TTF-TCNQ), metal-phthalocyanine, etc., and insulator films form aliphatic acids, etc.
Because of their nature as electric conductors or semiconductors, organic thin films having electroconductivity have potential use in electronics, opto-electronics, energy conversion and other fields of application. In particular, organic thin films having high conductivity hold promise for use in many applications including micro-wiring, striplines, sensors, display devices, memories and switching devices. However, no organic thin films that have high electroconductivity like metals are yet to be realized.
It has recently been proposed that an organic semiconductor or conductor film be prepared by a process that includes the steps of forming a film of an electron-donating organic compound such as a fulvalene containing a chalcogen atom in the molecule on an electrode substrate, and oxidizing said film by electrolysis (Unexamined Published Japanese Patent Application No. 289013/1989). According to a specific example disclosed in this prior patent, BEDT-TTF as an electron-donating organic compound was vacuum-deposited on a platinum-evaporated glass substrate, and gold was vacuum-deposited on the resulting BEDT-TTF film to form an electrode, and subsequently, the electrode was electrolytically oxidized with tetraethyl ammonium perchlorate used as an electrolyte, whereby a film having conductivity of 3.times.10.sup.-3 S/cm was obtained.
However, this organic thin film and all others that are produced by the conventional methods have conductivity lower than that of metals. Further, the temperature dependency of the conductivity of prior art conductive organic thin films does not show a metallic behavior.
Another problem with the prior art is that the melting point and the decomposition point of BEDT-TTF are so close to each other that it is extremely difficult to vaporize BEDT-TTF without generating decomposition products. If it were possible to prepare a vapor-deposited BEDT-TTF containing no decomposition products, an organic thin film having high conductivity could be produced by doping said BEDT-TTF film with an electron acceptor. However, no method has so far been proposed that is directed to forming a vapor-deposited BEDT-TTF film having such high quality and purity. Accordingly, there has been no report published on the possibility of using the vapor-deposited BEDT-TTF film to make an organic thin film that shows high conductivity or even superconductivity.